fix: switch HF Space to docker SDK, add Dockerfile

fix: expand requirements.txt from '.' to explicit deps for HF Space build

fix: shorten HF Space short_description to ≤60 chars

feat(deploy): add free Hugging Face Space entrypoint

feat(research): add low-cost models and web search toggle

feat(agent): add Tavily web search provider

feat(agent): support OpenRouter model routing

feat(agent): add checkpointed deep research service